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Transformations allow you to modify data as it flows through validation. This is especially useful for converting string parameters to numbers, parsing dates, normalizing data, and more.

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Basic Transformations

Use .transform() to convert validated data into a different format: 
import { z } from "zod";

const getUserEndpoint = endpointsFactory.buildVoid({
  input: z.object({
    id: z.string().transform((id) => parseInt(id, 10)),
  }),
  handler: async ({ input: { id }, logger }) => {
    logger.debug("id", typeof id); // number
    // id is now a number, not a string
  },
});

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Why Transformations?

Query parameters and path parameters always arrive as strings. Transformations let you convert them to the types your code expects: 
// GET /api/users?page=2&limit=50

const listUsersEndpoint = endpointsFactory.build({
  method: "get",
  input: z.object({
    page: z.string().transform(Number),
    limit: z.string().transform(Number),
  }),
  output: z.object({
    users: z.array(userSchema),
    page: z.number(),
    limit: z.number(),
  }),
  handler: async ({ input }) => {
    // page and limit are numbers, not strings
    const users = await db.users.find({
      skip: (input.page - 1) * input.limit,
      limit: input.limit,
    });
    return { users, page: input.page, limit: input.limit };
  },
});

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Common Transformation Patterns

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String to Number

z.object({
  id: z.string().transform((val) => parseInt(val, 10)),
  price: z.string().transform((val) => parseFloat(val)),
})
For coercion without explicit transformation, use z.coerce.number() which handles both strings and numbers.

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String to Boolean

z.object({
  active: z.string().transform((val) => val === "true"),
})

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Trimming and Normalizing

z.object({
  email: z.string().transform((val) => val.trim().toLowerCase()),
  username: z.string().transform((val) => val.trim()),
})

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Parsing JSON Strings

z.object({
  metadata: z.string().transform((val) => JSON.parse(val)),
})

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Combining Validation and Transformation

You can chain validators before and after transformations: 
import { z } from "zod";
import { defaultEndpointsFactory } from "express-zod-api";

const updateUserEndpoint = defaultEndpointsFactory.build({
  method: "patch",
  input: z.object({
    id: z
      .string()
      .regex(/^\d+$/, "ID must be numeric")
      .transform((id) => parseInt(id, 10))
      .refine((id) => id >= 0, "ID must be non-negative"),
  }),
  output: z.object({
    success: z.boolean(),
  }),
  handler: async ({ input }) => {
    // id is validated as string, transformed to number, then validated as number
    return { success: true };
  },
});

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Top-Level Transformations

You can transform the entire input object, useful for renaming fields or changing naming conventions: 
import camelize from "camelize-ts";
import { z } from "zod";

const endpoint = endpointsFactory.build({
  input: z
    .object({ user_id: z.string() })
    .transform((inputs) => camelize(inputs, /* shallow: */ true)),
  output: z.object({
    success: z.boolean(),
  }),
  handler: async ({ input: { userId }, logger }) => {
    logger.debug("user_id became userId", userId);
    return { success: true };
  },
});
For output transformations that need to appear in API documentation, use .remap() instead of .transform().

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Remapping for Documentation

The .remap() method (part of the Zod Plugin) transforms data while preserving schema information for documentation generation: 
import camelize from "camelize-ts";
import snakify from "snakify-ts";
import { z } from "zod";

const endpoint = endpointsFactory.build({
  input: z
    .object({ user_id: z.string() })
    .transform((inputs) => camelize(inputs, /* shallow: */ true)),
  output: z
    .object({ userName: z.string() })
    .remap((outputs) => snakify(outputs, /* shallow: */ true)),
  handler: async ({ input: { userId }, logger }) => {
    logger.debug("user_id became userId", userId);
    return { userName: "Agneta" }; // becomes "user_name" in response
  },
});

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Custom Field Mapping

You can also use .remap() with explicit field mappings: 
z.object({ user_name: z.string(), id: z.number() }).remap({
  user_name: "weHAVEreallyWEIRDnamingSTANDARDS",
  // "id" remains intact (partial mapping)
});

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Real-World Example

Here’s a complete example from the Express Zod API source showing transformation in action: 
import { z } from "zod";
import { ez, defaultEndpointsFactory } from "express-zod-api";

const updateUserEndpoint = defaultEndpointsFactory.build({
  method: "post",
  input: z.object({
    // Path parameter arrives as string
    id: z
      .string()
      .example("12")
      .transform((value) => parseInt(value, 10))
      .refine((value) => value >= 0, "should be greater than or equal to 0"),
    name: z.string().nonempty(),
    birthday: ez.dateIn(), // Transforms ISO string to Date
  }),
  output: z.object({
    name: z.string(),
    createdAt: ez.dateOut(), // Transforms Date to ISO string
  }),
  handler: async ({ input }) => {
    // input.id is a number
    // input.birthday is a Date
    return {
      name: input.name,
      createdAt: new Date("2022-01-22"),
    };
  },
});

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Type Safety with Transformations

TypeScript automatically infers the transformed types: 
const schema = z.object({
  id: z.string().transform(Number),
  tags: z.string().transform((s) => s.split(",")),
});

type Input = z.input<typeof schema>;
// { id: string; tags: string }

type Output = z.output<typeof schema>;
// { id: number; tags: string[] }
In your handler, input has the output type with all transformations applied.

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Best Practices

Transform at the Schema Level

Transformations in schemas are validated and documented. Avoid manual transformations in handlers.

Keep Transformations Simple

Complex logic should go in the handler, not in transformations.

Use Coercion When Appropriate

z.coerce.number() is cleaner than .transform(Number) for simple type coercion.

Validate Before and After

Add validation both before transformation (on the raw input) and after (on the transformed value).

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Next Steps

Dates

Learn about date handling with ez.dateIn() and ez.dateOut()

Refinements

Add custom validation logic

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